Making Nonsticky Surfaces of Sticky Materials: Self-Organized

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Making Non-sticky Surfaces of Sticky Materials: Self-organized Micro-texturing of Viscoelastic Elastomeric Layers by Tearing Sandip Patil, Tushar Deshpande, Nayantika Chaudhari, Yogesh R. G. Singh, Janhavi S. Raut, Yogesh M Joshi, and Ashutosh Sharma Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04389 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Making Non-Sticky Surfaces of Sticky Materials: Self-organized Microtexturing of Viscoelastic Elastomeric Layers by Tearing Sandip Patila, Tushar Deshpandea, Nayantika Chaudharia, Yogesh R. G. Singha, Janhavi Rautb, Yogesh M. Joshia and Ashutosh Sharmaa* [a*] Prof. Ashutosh Sharma Department of Chemical Engineering Indian Institute of Technology Kanpur-208016, U.P (India) E-mail: (([email protected])) Telephone: +91-512-2597026 Fax: +91-512-2590104 [b] Unilever R&D, 64 Main Road, Whitefield, Bangalore 560066, India

ABSTRACT: Fabrication of large area, multi-scale micro-textured surfaces engineered for anti-adhesion property remains a challenge. Compared to an elastic surface, viscoelastic solids show much higher surface stickiness, tack and adhesion owing to the increased contact area and energy dissipation. Here, we show a simple, low cost, large-area and high throughput method with roll-to-roll compatibility to fabricate multi-scale, rough micro-structures resistant to adhesion in a viscoelastic layer by controlled tearing of viscous film. Even a high adhesive strength viscoelastic solid layer, such as partially cured PDMS, is made non-sticky simply by its controlled tearing. The torn surface shows a fracture induced, self-organized leaf-like micro-pattern resistant to sticking. The topography and adhesion strength of these structures are readily tuned by changing the tearing speed and the film thickness. The micro-texture displays a spring-like recovery, low adhesive strength, and easy release property even under the high applied loads. Keywords: Polydimethylsiloxane (PDMS), Tearing, Micro-pattern, Adhesion energy, RMS roughness

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INTRODUCTION Anti-stick surfaces with angular micro-scale roughness are critical in many emerging applications such as micro- and nano-electromechanical systems (MEMS/NEMS),1–5 bioMEMS, and biological applications6,7 that require non-adhesive surface properties. Most nonadhering surfaces in nature are highly patterned with hierarchical structures.8 For example, self-cleaning lotus leaves have a complex micro- and nanoscopic surface architecture that can minimize the adhesion of dirt particles and water.9 Similarly, rough patterns on shark skin greatly reduce the water drag and adhesion, making the creature’s motion more energy efficient.10 All these examples indicate the importance of complex patterns at the surface of a material’s adhesion properties. To mimic such a surface over a large area by polymer imprinting11,12 replica molding,13–15 or hot embossing16 is a challenge because of the high cost and complexity of fabrication. Furthermore, all other traditional methods for generating micro/nano-textured

surfaces,

such

as

electric-field-induced

micro-patterning,17–20

electrospinning,21,22 chemical vapor deposition, and etching23 require a longer fabrication time. Also, fabrication by electron24 and ion25 beams to generate rough surfaces is effective only in very small areas and at very low throughputs. Other parallel techniques such as photolithography,26 nano-imprint

lithography,27,28

and

soft

lithography29–31

require

prefabrication masks to generate the micro-patterned rough surfaces. Other interesting approaches to micropattern generation are by surface instabilities such as wrinkling.32–34 Here, we report a low-cost, high-throughput method of fabricating a self-organized micropatterned rough surface by tearing out viscoelastic polydimethylsiloxane (PDMS) films on a large surface area (~600 mm2). Fracturing or cohesive failure by tearing produces two selfsimilar surfaces exhibiting micro-structures with multiscale roughness. The surface topography and roughness of the fractured surfaces are found to depend strongly on the film thickness and tearing speed. Our study shows that these self-organized micro-textures reduce


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the adhesive strength of a viscoelastic elastomer by more than an order of magnitude compared to similar smooth surfaces.

EXPERIMENTAL SECTION The PDMS (Sylgard 184, Dow Chemicals) used in the experiments contains an elastomer and a cross-linking agent. The elastomer and cross-linking agent were mixed in a ratio of 100:2 w/w in a clean glass beaker (2% cross-linked PDMS) and degassed under vacuum using a desiccator to remove trapped air from the solution. The prepared PDMS solution was then cast between two clean glass substrates by inserting spacers 40 µm or 80 µm in thickness and held tight by clamping the glass slide from both ends. After casting, the PDMS films were subsequently cured at 85°C for 48 hours.35–37 The cured 2% cross-linked PDMS films were torn using the setup shown in Figure 1a. As shown in the figure 1a, the bottom glass plate of dimensions 30 mm x 20 mm x 3 mm, was rigidly mounted on an anti-vibration platform. The mounted glass plate was separated from the top glass plate of dimensions 40 mm x 20 mm x 3 mm. The peeling position was 10 mm from the notch edge or from the adhesive layer edge. Tearing was performed at a controlled speed by applying a vertical force to the hanging edge of one of the glass substrates with micro motion controller equipment. The microscope image of the torn surface is shown in Figure 1b. The rheology of the 2% crosslinked PDMS was characterized by an oscillatory parallel plate rheometer (Anton Paar MCR-501). Figure 1c, shows the typical rheological response for storage modulus, G '( f ) and loss modulus, G "( f ) as a function of frequency, f ( Hz ) . Rheology response of prepared PDMS shows viscoelastic solid behaviour.38


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a

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Hanging Glass Plate

Viscoelstic PDMS film

b

Figure 1. Schematic of tearing setup for micro-pattern fabrication and rheology plot for 2% crosslink PDMS. (a) Schematic of cohesive zone formation during tearing in viscoelastic PDMS film. The inset shows cohesive crack profile. (b) Typical optical image of the micropatterned surface generated during tearing of viscoelastic PDMS film of thickness h = 20 µm (half of the total original thickness) for a tearing speed Vt = 100 µm/s. (c)Storage, G '( f ) and loss, G "( f ) moduli response as a function of frequency for the 2% cross-linked PDMS.

RESULTS AND DISCUSSION Viscoelastic soft solid PDMS films of thickness 40 µm or 80 µm glued and confined between two glass substrates were torn by separating one of the hanging glass substrates at a controlled speed of 1–500 µm/s, as shown in Figure 1a. During separation, the adhesive force between the PDMS film and the glass plate was strong, which produced a cohesive fracture in the 4 Environment ACS Paragon Plus

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PDMS film. Adhesion of viscoelastic films has been discussed elsewhere.39 The films with less than about 1% crosslinker are viscous liquid and the films with more than about 3% cross-linker are elastic solid. The most interesting case corresponds to the tearing of viscoelastic soft solids with comparable loss and storage moduli. This case, which is typical of pressure sensitive adhesives, engenders good adhesion. Figure 1a shows a schematic of the cohesive zone during film tearing. The onset of cohesive fracture in the viscoelastic 2% crosslink PDMS with comparable loss and storage moduli is governed by the formation of bubbles or cavities and finger-like cracks in the bulk of the film before complete fracturing.39,40 These pre-fracture formations are important for the generation of micropatterns on the fractured surfaces, as discussed by Patil et al.40 During fracture, the 2% crosslink PDMS film breaks into two pieces of nearly equal thickness (half of the total thickness). The material balance analysis was done of each glass plate by weight. It was found to be distributed into two layers with ± 1 % variation. Each piece of the 2% crosslink PDMS fracture surface has complimentry micropatterns of similar shape and size. A typical optical image of the micropattern generated by tearing is shown in Figure 1b. The viscoelastic liquid (0.5% crosslinker PDMS) and elastic solid (4% crosslinker PDMS) were also tested (results not shown). As expected, films with 0.5% crosslinker were viscous liquid in which no equilibrium pattern formation is seen on the fracture surfaces. On the other hand, 4% crosslinker elastic solid films have high fracture strength. Thus, tearing of such films required very high loads, leading to the adhesive failure from the plates. In any event, the highly crosslinked elastic films are not models of viscoelastic adhesives because their adhesive strength is already negligible. Thus, such films are not appropriate to study the loss of stickiness caused by the surface patterns induced by tearing. The formed micropatterns on the torn surfaces were examined by optical profilometry (Wyko NT-1100) to assess the surface topography and roughness. Topographical images of the micropatterns are shown in Figure 2a and b for films of thickness h = 20 µm and in Figure 2c and d for films of thickness h =40 µm at tearing 5 Environment ACS Paragon Plus

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speeds of Vt = 1 µm/s and 500 µm/s, respectively. The RMS roughness was obtained from the height profile and computed by using the ANSI B46.1 standard in optical profilometry software (VISION). RMS roughness is calculated as the Root Mean Square of a surface measured microscopic peaks and valleys. RMS roughness was obtained from.40 ܰ

1 ܴ‫ = ݍ‬ඩ ෍ሺܼ݅ − ܼܽ ሻ2 ܰ ݅=0

Here, Za is the average of the Z (height) values within the given area Zi is the Z value at a given point, and N is the number of sampled points within the given area. Figure 3 shows the roughness profiles at various tearing speeds for 20 and 40 µm film thicknesses (half of the total original thickness).


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Figure 2. Optical profilometry analysis. (a) h = 20 µm at Vt = 1 µm/s. (b) h = 20 µm at Vt = 500 µm/s. (c) h = 40 µm at Vt = 1 µm/s. (d) h = 40 µm at Vt = 500 µm/s. Red lines indicate cross section across the x-direction, the height of which is profiled below. Three-dimensional profiles signify the surface topography of the micro-patterned surface.

Figure 3. RMS roughness as a function of tearing speed Vt for film thicknesses of 20 µm and 40 µm (half of the total original thickness). The error bars shown in the figure are calculated by the standard deviation of five samples.

The detailed characterization of the fractured surfaces and the pre-facture instabilities leading to the final petal-like structures is presented by Patil et al.40 Morphology of the self-affine surfaces of the torn viscoelastic PDMS layers was characterized by power spectral density to access their fractal dimension. The fractal dimension of the surface was found close to 1.5, independent of the tearing speed (in the range of 1−500 µm/s). This value of the fractal dimension signifies that the height variations follow a Wiener process. The Wiener process correlates the self-affine or Brownian process in which average features of the surface do not change while zooming in.1,10 Interestingly, although the characteristic length scale of the pattern increased somewhat with increased tearing speed, it did not alter its original fractal dimension.40 Interestingly, the surface texture produced after tearing did not change appreciably even after two months, thus indicating severe viscous dissipation/plastic deformation with little if any residual elastic stresses. Thus, the adhesion energy maintained similar values long after tearing.


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The adhesive energy of the micro-textured surfaces was measured by a peel test as shown in Figure 4a.35,41,42 In the peel test, a silane-monolayer-coated flexible glass cover plate43 was brought into complete contact with the micro-patterned surface. The flexibility of the microscopic cover plate was chosen such that it undergoes minimal bending during peeling. Before peeling, the cover plate was pressed against the micro-patterned surface with an evenly distributed preload of 1 kg per 300 mm2, without which the micro-textured surface owing to its asperities did not make any significant contact, and the adhesion energy was nearly zero. The contact line with and without preload is shown in the figure 4b. On the other hand, smooth films did not require any preload to bring them in uniform contact. In fact, application of pre-load often pushed the cover plate inside the adhesive film and some material flowed out from the edges on to the top of the cover plate owing to its low stiffness. Thus, in all the experiments reported on the micro-textured surfaces, a fixed preload of 1 kg per 300 mm2 was applied and removed after 5 minutes. After removal of the preload, the cover plate was lifted vertically from its hanging edge at a constant slow peeling speed of 3 µm/s using a micromanipulator, as shown in Figure 4a. The force required for peeling was measured and recorded by a strain gauge force sensor. The adhesion strength or energy was evaluated by integrating the area under the force-displacement (F–∆) curve and dividing it by the contact area (300 mm2) between the cover plate and the adhesive surface. The details and interpretation of the peel test can be found elsewhere.35,41,42,44–46 Sets of such F–∆ plots are shown in Figure 5a and 5b for films of thickness h = 20 µm and 40 µm, respectively; the inset shows an F–∆ curve for a similar smooth film. a


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b With load Without load

No load

Figure 4. (a) Schematic of peeling setup. Displacement (∆) controlled by motorized micromanipulator to peel the glass plate from the micro-patterned surface. Speed of detachment was 3 µm/s at a cover plate rigidity D = 0.02 Nm. (b) Microscopic evolution of contact area between the glass plate and micro-patterned surface; black boundary indicates contact area.

The maximum force in the F–∆ plots represents the maximum crack initiation force, after which crack propagation begins at the interface between the glass plate and the micropatterned surface. These plots indicate that compared to a smooth surface, a microtextured torn surface can display much lower crack-initiation force and a shorter force plateau for the crack propagation (insets). Figure 5a shows that the F–∆ curves for h = 20 µm and Vt = 1 and 10 µm/s differ significantly from the others. In these two cases, the crack initiation force is almost the same as that of a smooth film, as shown in the inset in Figure 5a. However, unlike the case of a smooth film, as soon as a crack is initiated, the force suddenly decreases to zero, causing complete detachment of the glass plate from the adhesive film. At all the other tearing speeds, the crack initiation force in the micro-patterned surfaces is much lower than that in smooth films. For example, the crack initiation force decreases from 0.06 N to 0.02 N ( h = 20 µm) for the films torn at higher speeds, Vt from 100 to 500 µm/s. For a thicker 40 µm thick film, (Figure 5b) the crack initiation forces appear non-monotonic, unlike those in 20µm thick films. The non-monotonic behavior and the shorter plateau of F–∆ curves for the fractured surface of the 40 µm film are most likely owing to a sudden detachment caused 9 Environment ACS Paragon Plus

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by a spring-like action of the sharp aspirities of higher roughness, which engenders a highly non-uniform contact area between the coverslip and the fracture surface. We have shown (Supporting video) that after removing the preload, coverslip just jumps up to sit on the sharp asperities of the fracture surface with a non-uniform contact. Also, the nonuniform weak contact spots drive the discontinuous crack initiation and propagation which is observed in figure 5b. Another observation is of the surface roughness generated on the micro-patterned surfaces. The effect of surface roughness on adhesion has been discussed theoretically and experimentally for perfectly elastic47–49 and viscoelastic systems.50–52 The surface roughness creates in-homogeneity on the micro-patterned surfaces, reducing the interaction between the cover plate and the adhesive film. Because of the surface roughness, the force required to propagate the crack is less for the micro-patterned surface than for the smooth surface. I

0.16

0.12 0.06

II III 0.0

0.8

1.6

2.4

0.00

Vt = 1 µm/s

∆ (mm)

Vt = 10 µm/s Vt = 100 µm/s

0.08

Vt = 300 µm/s Vt = 500 µm/s

0.04 0.00 0.0 0.024

0.2 b

∆ (mm)

0.4

Vt = 10 µm/s Vt = 100 µm/s

0.018

0.6

0.18

Vt = 1 µm/s F (N)

F(N)

0.12

F(N)

0.18

a

Vt = 300 µm/s

0.12 0.06

Vt = 500 µm/s

F (N)

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0.0 0.6 1.2 1.8 2.4 ∆ (mm)

0.012

0.006

0.000 0.0

0.4

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∆ (mm)


2.4

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Figure 5. Force–Displacement (F–∆) plots for film thicknesses of (a) h = 20 µm and (b) h = 40 µm. Insets show the force–displacement curves for a similar smooth film at a peeling speed of 3 µm/s.

The relationship between the total adhesion energy, G and tearing speeds, Vt is shown in Figure 6. The adhesion energy was evaluated for three different regions, as shown in the inset of Figure 5a: (I) total energy, G (II) crack initiation energy, GI and (III) crack propagation energy, GP. In the discussion below, we focus on the total adhesion energy, G . The total adhesion strength of the smooth viscoelastic PDMS surface was found to be 428 ± 26 mJ/m2 for h = 20 µm and 496 ± 19 mJ/m2 for h = 40 µm. The same thickness film was torn at various tearing speed; it shows interesting dynamics of the reduction in adhesion strength compared to smooth, controlled film. From the adhesion energy plots in Figure 6a, the total adhesion strength of the fracture surfaces is ~2 to 17 times smaller than that of a smooth control surface. This is rather impressive considering that the adhesion strength of the textured surfaces is measured at a low peel speed (3 µm/s) after application of a considerable pre-load.


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Figure 6. Adhesion energy as function of tearing speed. (a) Total adhesion energy. (b) Crack initiation energy. (c) Crack propagation energy. Film thicknesses were h = 20 µm and h = 40 µm. The error bars shown in the figure are calculated by the standard deviation of five samples.

The decrease in adhesion energy G, (total energy) is directly related to the decreased contact area, A and RMS roughness, Rq. Figure 7a shows that the ratio of energy to contact area ( GAo / Go A ) versus the tearing speed lies between 0.3 < GAo / Go A < 0.95, where G and

Go are the adhesion energies of the micro-patterned and smooth films, respectively, and A and Ao are the contact areas of the cover plate with the textured and smooth films, respectively. In case of complete contact ( GAo / Go A ~ 1), the total adhesion energy becomes equal to its flat surface adhesion energy. In the case of micro-textured surfaces, when,

GAo / Go A < 1, the adhesion energy falls quickly as the roughness increases, as shown in 12 Environment ACS Paragon Plus

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Figure 7b. The surface roughness is one of the key factors for the decrease in A and G . The roughness factor can be expressed as the ratio of the actual and projected contact areas,

A / Ao .2 As the scale of the roughness become finer and approaches the molecular scale, the surface essentially behaves as a smooth surface, as observed at Rq< 0.48 µm in Figure 7b. At Rq> 0.48, adhesive interactions dominate only at the contacting asperities. 1.0 a

h = 20 µm h = 40 µm

0.9

GAo/GoA

0.8 0.7 0.6 0.5 0.4 0.3 0

100

200 300 Vt (µm/s)

400

500

b

200

2

h = 20 µm h = 40 µm

150

Gtotal (mJ/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 50 0

0.4

0.8

1.2

1.6

2.0

Rq (µm)

Figure 7. (a) Total adhesion energy as a function of contact area ratio for films of thickness 20 µm (squares) and 40 µm (circles). (b) Dependency of RMS roughness on adhesion energy for thicknesses of 20 µm (squares) and 40 µm (circles). The error bars shown in the figure are calculated by the standard deviation of five samples. The total adhesion energy decreases quickly with an increase in surface roughness for Rq< 0.6 µm. If Rq is increased further to greater than 0.6 µm, the adhesion energy values do not change significantly. The total adhesion energy decreases from 185 to 20 mJ/m2 as the roughness increases from 0.4 to 1.8 µm. All of the above adhesion energy analysis was performed under the application of a 1kg preload over a 300 mm2 contact area. Afterward, a 13 Environment ACS Paragon Plus

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peel test experiment was also performed under various preloads and under hand pressure from the top over the same contact area. The resulting adhesion energy was calculated, as shown in Figure 8 for a 40 µm film thickness at a 500 µm/s tearing speed.

Figure 8. Adhesion energy as a function of applied preload. Plots represent the total adhesion energy (squares), crack propagation energy (circles), and crack initiation energy (triangles). Film thickness was 40 µm; tearing speed was 500 µm/s.

The data obtained from adhesion tests at various preloads reveal that the adhesion energy does not vary significantly with the preload. These results show that the torn microtextured surfaces exhibit a spring-like recovery after the removal of the pre-load. This springlike behavior is clearly visible in the video provided in the supporting material. The above discussion also provides a clue to an interesting everyday observation. The adhesion of pressure-sensitive adhesives decreases on their second application after they are applied once and peeled. In actual appplications, this is partly owing to dirtying of the surface of the adhesive, and partly, as we propose here, because of the surface roughness that could be generated by the adhesive layer’s cohesive failure or by the self-organized instabilities in peeling. To explore this further qualitatively, we selected a commercial pressure-sensitive adhesives tape (Cello tape, India, thickness ~20 µm). The pressure-sensitive adhesive tape was first stuck to and then removed from a rough, but clean solid surface. A peel test on the removed tape was immmediately performed using another clean glass surface while taking care to prevent any contamination. Figure 9 shows the force-displacement curves for the first and second use of the adhesive tape. The force-displacement analysis shows that the smooth 14 Environment ACS Paragon Plus

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surface of the first use adhesive tape produces a higher crack initiation force and more extended crack propagation plateau. However, for the once-used tape, less crack initiation force is required and the crack propagation plateau is shorter. These features are similar to the peel test results for the torn micro-textured viscoelastic PDMS surfaces as shown in the Figure 5. The force-displacement curves also show that the adhesion strength of the once used adhesive tape is about half that of the first use smooth adhesive tape (Figure 9). 2

Fresh adhesive tape (G = 8785 mJ/m ) 2 Fracture adhesive tape (G = 4844 mJ/m )

0.6 0.5

F (N)

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0.4 0.3 0.2 0.1 0.0

0

2

4

6

8

10

∆ (mm)

Figure 9. Force-Displacement (F–∆) plots for commercial adhesive tape at 3 µm/s peeling speed. Total adhesive strength for fresh adhesive tape is 8785±135 mJ/m2, and that for fractured adhesive tape is 4844±109 mJ/m2.

The peel test results reported here for viscoelastic PDMS and commercial adhesive tape show good agreement with those of the previous studies47,48,53–55 in terms of the decrease in adhesion energy with increasing surface roughness. Earlier studies reported an approximately two-fold reduction in adhesion energy with a randomly rough surface of polystyrene–polyvinylpyrrolidone diblock polymer.53 For periodically patterned rough surfaces, such as flat tips with rounded edges and concave patterns, the adhesion forces were ~3-5 times smaller than those for a control PDMS surface.54 Furthermore, Nosonovsky et al.2 explained that elastic surfaces with very sharp edges, such as sawtooth and conical (triangular) patterns, behave like non-adhesive surfaces. Similarly, a recent study by


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Bhushan55 showed that the rough surface with micro ribs on shark skin also reduces the adhesion energy considerably. All the experiments reported thus far on the reduction in adhesion strength as a function of roughness have been performed on randomly rough surfaces created by fracture. To understand more about periodical roughness profile on adhesion strength we also performed adhesion experiments on periodically pattern surfaces such as sharp edge (triangular) and curved surface (sine wave ) geometries. The motivation for this part of the work in the present study is to further understand the adhesive properties of the self-created micropatterned surfaces by examing the effect of geometry of the aspirities. Triangular and sine wave patterns with different pattern periodicity λ=50, 100 and 150µm were fabricated on 2% crosslink PDMS film (1 mm total thickness) film (2% crosslinked) by replica molding process. Increase in pattern periodicity also translates into decrease in the fractional contact area. The procedure of replica molding process using maskless photolithography system is reported elsewhere.31 Detail surfaces analysis of these surfaces is shown in Figure 10. The peel tests were performed on these periodical pattern rough surfaces. Hand pressing made precontact between patterns and glass cover plate. A similar set of preload experiments are carried out in self-assembled micro-pattern surfaces in the previous section. Figure 11 shows adhesion energies for triangular and sine wave micropattern surfaces at 3 µm/s peeling velocity. Peeling direction was chosen parallel and perpendicular to the periodicity of the patterns. The adhesion strength of patterned surfaces were compared with that of smooth PDMS film (2% crosslinked) for same film thickness. The peel test results show good agreement with our finding in terms reduction in adhesion energy with increasing periodicity of the patterns. The total adhesion energies of triangular and sine wave patterns were ~2-14 times less than control smooth PDMS surface. Moreover similar to our findings, the adhesion strength decreases with increase in pattern periodicity from 50, 100 and 150 µm for both triangular and sine wave patterns. These results support our observation of a decrease in 16 Environment ACS Paragon Plus

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adhesion energy for self-created micropattern surfaces with a decrease in contact area between the cover plate and micropattern surfaces.

Figure 10. Optical profilometry images for periodically pattern surfaces. (a) Sharp edge (triangular) surface pattern, λ=100µm (b) Curved surface (sine wave) pattern λ=100µm. The height (z)-profiling was carried at a cross section indicated by white lines across the xdirection. Three-dimensional profiles depict fine surface topographies of sharp edge and curved surfaces.


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Figure 11. Total adhesion energy as a function of pattern periodicity. Solid symbols represent results of peeling direction parallel to the pattern periodicity and hollow symbol represent adhesion energies for peeling direction perpendicular to pattern periodicity. Peel tests were performed over 300mm2 contact area with 3µm/s detachment speed.

This work provides an alternative strategy for tuning the adhesive properties of a viscoelastic soft solid glue by control of its surface texture. This method of adhesion control by micro-texturing of viscoelastic layers by simple tearing may also find applications in MEMS/NEMS, biomedical devices and other applications where soft solid surfaces with low adhesion are desired.

CONCLUSIONS Making non-sticky surfaces of an otherwise sticky material by appropraite surface microtexturing is an interesting aspect of beyond-the-material adhesion. In this article, we show a simple, low-cost, large-area, high-throughput fabrication method for creating surface micropatterns of different length scales, RMS roughness and adhesive strengths in soft solid viscoelastic films. Both the surface roughness features and the adhesion can be controlled by driving the cohesive failure (tearing or fracturing) of the film at different tearing speed. Tearing of the soft solid viscoelastic films engenders the sharp tipped, robust asperities that act as micro-springs, which act to minimize the contact area immediately after the applied pre-load for the adhesion is removed. This self-organized microtextures engender significantly low adhesion energies, i.e., about 2–17 times less than that of a smooth surface of the same material. Further, the adhesion energy in this range can be tuned by varying the film thickness and the tearing speed which change the surface texture. The adhesion energy decreases rapidly with an initial increase in the surface roughness and later becomes invariant after reaching an intermediate roughness value. 18 Environment ACS Paragon Plus

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SUPPORTING INFORMATION

Real-time video for spring-like behavior testing on micropattern surfaces.

ACKNOWLEDGMENTS The authors also acknowledge Unilever project grant MA-2014-01638 to IIT Kanpur for this research work and the Department of Science and Technology, India through its grants to the Thematic Unit of Excellence on Soft Nanofabrication with Application in Energy and Bioplatform at IIT Kanpur for various characterization facilities. We are also thankful to Prof. Utpal Das and Dr. Ramesh Sonkar for providing optical profilometry facility.


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Reduction

and

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Table of Contents Fabrication of large area, multi-scale micro-textured surfaces engineered for anti-adhesion property remains a challenge. Compared to an elastic surface, viscoelastic solids show much higher surface stickiness, tack and adhesion owing to the increased contact area and energy dissipation. Here, we show a simple, low cost, large-area and high throughput method with roll-to-roll compatibility to fabricate multi-scale, rough micro-structures resistant to adhesion in a viscoelastic layer. Even a high adhesive strength viscoelastic solid layer, such as partially cured PDMS, is made non-sticky simply by its controlled tearing. The torn surface shows a fracture induced, self-organized leaf-like micro-pattern resistant to sticking. The topography and adhesion strength of these structures are readily tuned by changing the tearing speed and the film thickness. The micro-texture displays a spring-like recovery, low adhesive strength and easy release property even under high-applied loads. Keywords: Polydimethylsiloxane (PDMS), Tearing, Micro-pattern, Adhesion energy, RMS roughness Authors: Sandip Patila, Tushar Deshpandea, Nayantika Chaudharia, Yogesh R. G. Singha, Janhavi Rautb, Yogesh M. Joshia and Ashutosh Sharmaa* Title Making non-sticky surfaces of sticky materials by self-organized micro-texturing of viscoelastic layers by tearing 200

a

Total Energy

160 2

G (mJ/m )

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h = 20 µm h = 40 µm

120 80 40 0

0

100

200

300

400

500

Tearing speed (µm/s)


Making Nonsticky Surfaces of Sticky Materials: Self-Organized

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